Formation and Metabolism of Prebeta-Migrating, Lipid-Poor Apolipoprotein A-I

The rationale for using apoA-I as a clinical marker of cardiovascular risk

An inverse relationship between the concentration of high-density lipoprotein (HDL) cholesterol and the risk of developing cardiovascular is well established. There are several documented functions of HDLs that may contribute to a protective role of these lipoproteins. These include the ability of HDLs to promote the efflux of cholesterol from macrophages and foam cells in the artery wall and to anti-inflammatory/antioxidant properties of these lipoproteins. The fact that the main apolipoprotein of HDLs, apoA-I, plays a prominent role in each of these functions adds support to the view that apoA-I should be measured as a component of the assessment of cardiovascular risk in humans.

HDL from CETP-deficient subjects shows enhanced ability to promote cholesterol efflux from macrophages in an apoE- and ABCG1-dependent pathway

Genetic deficiency or inhibition of cholesteryl ester transfer protein (CETP) leads to a marked increase in plasma levels of large HDL-2 particles. However, there is concern that such particles may be dysfunctional in terms of their ability to promote cholesterol efflux from macrophages. Recently, the ATP-binding cassette transporter ABCG1, a macrophage liver X receptor (LXR) target, has been shown to stimulate cholesterol efflux to HDL. We have assessed the ability of HDL from subjects with homozygous deficiency of CETP (CETP-D) to promote cholesterol efflux from macrophages and have evaluated the role of ABCG1 and other factors in this process. CETP-D HDL-2 caused a 2- to 3-fold stimulation of net cholesterol efflux compared with control HDL-2 in LXR-activated macrophages, due primarily to an increase in lecithin:cholesterol acyltransferase-mediated (LCAT-mediated) cholesteryl ester formation in media. Genetic knockdown or overexpression of ABCG1 showed that increased cholesterol efflux to CETP-D HDL was ABCG1 dependent. LCAT and apoE contents of CETP-D HDL-2 were markedly increased compared with control HDL-2, and increased cholesterol esterification activity resided within the apoE-HDL fraction. Thus, CETP-D HDL has enhanced ability to promote cholesterol efflux from foam cells in an ABCG1-dependent pathway due to an increased content of LCAT and apoE.

Gene expression changes in foam cells and the role of chemokine receptor CCR7 during atherosclerosis regression in ApoE-deficient mice

Atherosclerosis regression is an important clinical goal. In previous studies of regression in mice, the rapid loss of plaque foam cells was explained by emigration to lymph nodes, a process reminiscent of dendritic cells. In the present study, plaque-containing arterial segments from apoE-/- mice were transplanted into WT recipient normolipidemic mice or apoE-/- mice. Three days after transplant, in the WT regression environment, plaque size decreased by approximately 40%, and foam cell content by approximately 75%. In contrast, both parameters increased in apoE-/- recipients. Foam cells were isolated by laser capture microdissection. In WT recipients, there were 3- to 6-fold increases in foam cells of mRNA for liver X receptor alpha and cholesterol efflux factors ABCA1 and SR-BI. Although liver X receptor alpha was induced, there was no detectable expression of its putative activator, peroxisome proliferator-activated receptor gamma. Expression levels of VCAM or MCP-1 were reduced to 25% of levels in pretransplant or apoE-/- recipient samples, but there was induction at the mRNA and protein levels of chemokine receptor CCR7, an essential factor for dendritic cell migration. Remarkably, when CCR7 function was abrogated in vivo by treatment of WT recipients with antibodies to CCR7 ligands CCL19 and CCL21, lesion size and foam cell content were substantially preserved. In summary, in foam cells during atherosclerosis regression, there is induction of CCR7 and a requirement for its function. Taken with the other gene expression data, these results in vivo point to complex relationships among the immune system, nuclear hormone receptors, and inflammation during regression.

Kinetics of prebeta1 HDL and alphaHDL in type II diabetic patients

BACKGROUND

The aim of this study was to analyze the recycling of high density lipoprotein (HDL) in six type II diabetic patients compared with six control subjects by endogenous labelling of apolipoprotein A-I (Apo A-I) with stable isotope Apo A.

MATERIALS AND METHODS

The -I-HDL kinetics were performed by infusion of (5.5.5-(2)H3)-leucine for 14 h. The prebeta1 and alphaHDL were separated by gel filtration fast protein liquid chromatrography system (FPLC). Kinetics of isotopic enrichment of Apo A-I were analyzed with a multi-compartmental model software (SAAM II, SAAM Institute, Seattle, WA).

RESULTS

Plasma Apo A-I concentration was decreased in patients with type II diabetes as a result of a decrease in Apo A-I-alphaHDL (P < 0.05). Diabetic patients were also characterized by an increased relative contribution of Apo A-I in prebeta1 HDL (18.3 +/- 2.8% vs 11.9 +/- 3.7%, P < 0.01). The synthetic rate of prebeta1 HDL was slightly increased in diabetic patients compared with control (NS) and an increase of recycling rate of alpha to prebeta1 HDL was observed (11.67 +/- 3.14 d(-1) vs 7.09 +/- 4.51 d(-1), P < 0.05). The clearance rate of Apo A-I was higher in diabetic patients (P < 0.05 for Apo A-I-prebeta1 HDL and P < 0.005 for Apo A-I-alphaHDL).

CONCLUSION

This study suggests that the usual increase in prebeta1 HDL in type II diabetic patients is mainly related to an increased conversion rate of alpha to prebeta1 HDL.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Lack of ABCA1 considerably decreases brain ApoE level and increases amyloid deposition in APP23 mice

ABCA1 (ATP-binding cassette transporter A1) is a major regulator of cholesterol efflux and high density lipoprotein (HDL) metabolism. Mutations in human ABCA1 cause severe HDL deficiencies characterized by the virtual absence of apoA-I and HDL and prevalent atherosclerosis. Recently, it has been reported that the lack of ABCA1 causes a significant reduction of apoE protein level in the brain of ABCA1 knock-out (ABCA1-/-) mice. ApoE isoforms strongly affect Alzheimer disease (AD) pathology and risk. To determine further the effect of ABCA1 on amyloid deposition, we used APP23 transgenic mice in which the human familial Swedish AD mutant is expressed only in neurons. We demonstrated that the targeted disruption of ABCA1 increases amyloid deposition in APP23 mice, and the effect is manifested by an increased level of Abeta immunoreactivity, as well as thioflavine S-positive plaques in brain parenchyma. We found that the lack of ABCA1 also considerably increased the level of cerebral amyloid angiopathy and exacerbated cerebral amyloid angiopathy-related microhemorrhage in APP23/ABCA1-/- mice. Remarkably, the elevation in parenchymal and vascular amyloid in APP23/ABCA1-/- mice was accompanied by a dramatic decrease in the level of soluble brain apoE, although insoluble apoE was not changed. The elevation of insoluble Abeta fraction in old APP23/ABCA1-/- mice, accompanied by a lack of changes in APP processing and soluble beta-amyloid in young APP23/ABCA1-/- animals, supports the conclusion that the ABCA1 deficiency increases amyloid deposition. These results suggest that ABCA1 plays a role in the pathogenesis of parenchymal and cerebrovascular amyloid pathology and thus may be considered a therapeutic target in AD.

Increased cellular cholesterol efflux in glycogen storage disease type Ia mice: A potential mechanism that protects against premature atherosclerosis

Glycogen storage disease type Ia (GSD-Ia) patients manifest a pro-atherogenic lipid profile but are not at elevated risk for developing atherosclerosis. Serum phospholipid, which correlates positively with the scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux, and apolipoprotein A-IV and E, acceptors for ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol transport, are increased in GSD-Ia mice. Importantly, sera from GSD-Ia mice are more efficient than sera from control littermates in promoting SR-BI- and ABCA1-mediated cholesterol effluxes. As the first step in reverse cholesterol transport, essential for cholesterol homeostasis, these observations provide one explanation why GSD-Ia patients are apparently protected against premature atherosclerosis.

Hypertriglyceridemia is associated with preβ-HDL concentrations in subjects with familial low HDL

Prebeta-HDL particles act as the primary acceptors of cellular cholesterol in reverse cholesterol transport (RCT). An impairment of RCT may be the reason for the increased risk of coronary heart disease (CHD) in subjects with familial low HDL. We studied the levels of serum prebeta-HDL and the major regulating factors of HDL metabolism in 67 subjects with familial low HDL and in 64 normolipidemic subjects. We report that the subjects with familial low HDL had markedly reduced prebeta-HDL concentrations compared with the normolipidemic subjects (17.4 +/- 7.2 vs. 23.4 +/- 7.8 mg apolipoprotein A-I/dl; P < 0.001). A positive correlation was observed between prebeta-HDL concentration and serum triglyceride (TG) level (r = 0.334, P = 0.006). In addition, serum TG level was found to be the strongest predictor of prebeta-HDL concentration in subjects with familial low HDL. The activities of cholesteryl ester transfer protein and hepatic lipase were markedly increased in subjects with familial low HDL without a significant correlation to prebeta-HDL concentration. Our results support the hypothesis that impaired RCT is one mechanism behind the increased risk for CHD in subjects with familial low HDL.

Apolipoprotein structural organization in high density lipoproteins: belts, bundles, hinges and hairpins

PURPOSE OF REVIEW

To summarize recent advances towards an understanding of the three-dimensional structures of the apolipoprotein components of HDL with a specific focus on high resolution models of apolipoprotein A-I.

RECENT FINDINGS

Since the primary sequence was first reported, various models have been advanced for the structure of apolipoprotein A-I, the major protein constituent of HDL, in its lipid-free and lipid-bound forms. Unfortunately, the generation of experimental data capable of distinguishing among the competing models has lagged far behind. However, recent experimental strategies, including X-ray crystallography, applications of resonance energy transfer and mass spectrometry, have combined with sophisticated theoretical approaches to develop three-dimensional structural models of apolipoprotein A-I with previously unavailable resolution.

SUMMARY

The recent synergy of sophisticated computer modeling techniques with hard experimental data has generated new models for apolipoprotein A-I in certain subclasses of HDL produced in vitro. The challenge now is to adapt and test these models in the more complex forms of HDL isolated directly from human plasma.

Biogenesis and speciation of nascent apoA-I-containing particles in various cell lines

It is generally thought that the large heterogeneity of human HDL confers antiatherogenic properties; however, the mechanisms governing HDL biogenesis and speciation are complex and poorly understood. Here, we show that incubation of exogenous apolipoprotein A-I (apoA-I) with fibroblasts, CaCo-2, or CHO-overexpressing ABCA1 cells generates only alpha-nascent apolipoprotein A-I-containing particles (alpha-LpA-I) with diameters of 8-20 nm, whereas human umbilical vein endothelial cells and ABCA1 mutant (Q597R) cells were unable to form such particles. Interestingly, incubation of exogenous apoA-I with either HepG2 or macrophages generates both alpha-LpA-I and prebeta1-LpA-I. Furthermore, glyburide inhibits almost completely the formation of alpha-LpA-I but not prebeta1-LpA-I. Similarly, endogenously secreted HepG2 apoA-I was found to be associated with both prebeta1-LpA-I and alpha-LpA-I; by contrast, CaCo-2 cells secreted only alpha-LpA-I. To determine whether alpha-LpA-I generated by fibroblasts is a good substrate for LCAT, isolated alpha-LpA-I as well as reconstituted HDL [r(HDL)] was reacted with LCAT. Although both particles had similar V(max) (8.4 vs. 8.2 nmol cholesteryl ester/h/microg LCAT, respectively), the K(m) value was increased 2-fold for alpha-LpA-I compared with r(HDL) (1.2 vs. 0.7 microM apoA-I). These results demonstrate that 1) ABCA1 is required for the formation of alpha-LpA-I but not prebeta1-LpA-I; and 2) alpha-LpA-I interacts efficiently with LCAT. Thus, our study provides direct evidence for a new link between specific cell lines and the speciation of nascent HDL that occurs by both ABCA1-dependent and -independent pathways.

ABCG1: how critical for cholesterol transport?

In the current issue of Cell Metabolism, Kennedy et al. (2005) have extended our understanding of the ABCG1 transporter. Their studies demonstrate that, at least in macrophages, ABCG1 is responsible for much of the cholesterol efflux that utilizes mature HDL as an acceptor.

Kinetic stabilization and fusion of apolipoprotein A-2:DMPC disks: comparison with apoA-1 and apoC-1

Denaturation studies of high-density lipoproteins (HDL) containing human apolipoprotein A-2 (apoA-2) and dimyristoyl phosphatidylcholine indicate kinetic stabilization. Circular dichroism (CD) and light-scattering melting curves show hysteresis and scan rate dependence, indicating thermodynamically irreversible transition with high activation energy E(a). CD and light-scattering data suggest that protein unfolding triggers HDL fusion. Electron microscopy, gel electrophoresis, and differential scanning calorimetry show that such fusion involves lipid vesicle formation and dissociation of monomolecular lipid-poor protein. Arrhenius analysis reveals two kinetic phases, a slower phase with E(a,slow) = 60 kcal/mol and a faster phase with E(a,fast) = 22 kcal/mol. Only the fast phase is observed upon repetitive heating, suggesting that lipid-poor protein and protein-containing vesicles have lower kinetic stability than the disks. Comparison of the unfolding rates and the melting data recorded by differential scanning calorimetry, CD, and light scattering indicates the rank order for the kinetic disk stability, apoA-1 > apoA-2 > apoC-1, that correlates with protein size rather than hydrophobicity. This contrasts with the tighter association of apoA-2 than apoA-1 with mature HDL, suggesting different molecular determinants for stabilization of model discoidal and plasma spherical HDL. Different effects of apoA-2 and apoA-1 on HDL fusion and stability may reflect different metabolic properties of apoA-2 and/or apoA-1-containing HDL.

Influence of ApoA-I structure on the ABCA1-mediated efflux of cellular lipids

The influence of apolipoprotein (apo) A-I structure on ABCA1-mediated efflux of cellular unesterified (free) cholesterol (FC) and phospholipid (PL) is not well understood. To address this issue, we used a series of apoA-I mutants to examine the contributions of various domains in the molecule to ABCA1-mediated FC and PL efflux from mouse J774 macrophages and human skin fibroblasts. Irrespective of the cell type, deletion or disruption of the C-terminal lipid-binding domain of apoA-I drastically reduced the FC and PL efflux ( approximately 90%), indicating that the C-terminal amphipathic alpha-helix is required for high affinity microsolubilization of FC and PL. Deletion in the N-terminal region of apoA-I also reduced the lipid efflux ( approximately 30%) and increased the K(m) about 2-fold compared with wild type apoA-I, whereas deletion of the central domain (Delta123-166) had no effect on either K(m) or V(max). These results indicate that ABCA1-mediated lipid efflux is relatively insensitive to the organization of the apoA-I N-terminal helix-bundle domain. Alterations in apoA-I structure caused parallel changes in its ability to bind to a PL bilayer and to induce efflux of FC and PL. Overall, these results are consistent with a two-step model for ABCA1-mediated lipid efflux. In the first step, apoA-I binds to ABCA1 and hydrophobic alpha-helices in the C-terminal domain of apoA-I insert into the region of the perturbed PL bilayer created by the PL transport activity of ABCA1, thereby allowing the second step of lipidation of apoA-I and formation of nascent high density lipoprotein particles to occur.

A review of CETP and its relation to atherosclerosis

Although the atheroprotective role of HDL cholesterol (HDL-c) is well documented, effective therapeutics to selectively increase plasma HDL-c levels are not yet available. Recent progress in unraveling human HDL metabolism has fuelled the development of strategies to decrease the incidence and progression of coronary artery disease (CAD) by raising HDL-c. In this quest for novel drugs, cholesteryl ester transfer protein (CETP) represents a pivotal target. The role of this plasma protein in HDL metabolism is highlighted by the discovery that genetic CETP deficiency is the main cause of high HDL-c levels in Asian populations. The use of CETP inhibitors to effectively increase HDL-c concentration in humans was recently published and data with regard to the effect on human atherosclerosis are expected shortly. This review discusses the potential of CETP inhibitors to protect against atherosclerosis in the context of the current knowledge of CETP function in both rodents and humans.

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221–232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.

Endothelial lipase-modified high-density lipoprotein exhibits diminished ability to mediate SR-BI (scavenger receptor B type I)-dependent free-cholesterol efflux

Endothelial lipase (EL) is a phospholipase with little triacylglycerol lipase activity. To assess structural and functional properties of EL-HDL (EL-modified high-density lipoprotein), HDL was incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding human EL. After re-isolation of HDL by ultracentrifugation, TLC and HPLC analyses revealed that EL-HDL was markedly depleted in phosphatidylcholine and enriched in lyso-phosphatidylcholine compared with LacZ-HDL (control HDL) incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding beta-galactosidase. The EL-HDL was enriched in non-esterified fatty acids and, as revealed by lipid electrophoresis, was more negatively charged than control HDL. The HDL particle size as well as the total cholesterol, free cholesterol and triacylglycerol content of HDL were not significantly altered after EL modification. The ability of EL-HDL to mediate 3H-cholesterol efflux from SR-BI (scavenger receptor B type I) overexpressing Chinese-hamster ovary cells was impaired and markedly lower compared with LacZ-HDL at HDL concentrations of 100 microg/ml and above. Studies with 125I-labelled HDL showed almost unaltered binding affinity (K(m) values) and a slightly but significantly decreased binding capacity (B(max) values) of EL-HDL to SR-BI, compared with LacZ-HDL. The ATP-binding-cassette transporter A1-dependent cholesterol and phospholipid effluxes were not affected by EL modification. From these results, we concluded that EL modification alters chemical composition and physical properties of HDL, resulting in its decreased binding capacity to SR-BI and a diminished ability to mediate SR-BI-dependent cholesterol efflux.

Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins

Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.